This invention relates generally to semiconductor light sources, and more particularly to high output power multiple wavelength devices.
Semiconductor laser sources have proven useful in many applications, ranging from optical storage (DVD/Blue-Ray), to fiber-optic telecommunications, to optical pumps for solid state lasers. Their advantages are their small size, relatively high output power and brightness, and ability to control the wavelength. However, the total output power one can obtain from a single laser element in a single spatial mode has been limited to about 1 to a few Watts, depending on the wavelength and material. To obtain higher output power, certain sacrifices generally must be made.
The simplest way to get more power out of a semiconductor laser source is to make the optical waveguide wider. In these “broad area” structures, the output power is much higher, but the light is coming from a larger source and multiple lateral modes are excited. Therefore it is generally not possible to focus the light into a small spot, or collimate the beam with precision. To get even higher power, arrays of broad area lasers are fowled into bars and can generate hundreds of watts of power—limited only by the ability to extract the heat. The light however is coming from a very large area and the device acts much more like a light bulb than a laser, with no spatial coherence. Nevertheless, such “bars” can be used to pump solid state crystals like Nd/YAG to get secondary lasing in the crystals. This secondary lasing can be single mode with high brightness. Alternatively, such bars can be coupled to multimode fibers that can pump a double-clad fiber laser, once again leading to secondary lasing in a single spatial mode. These fiber lasers have recently been very successful displacing high power YAG or CO2 lasers for material and processing applications. The issue with using semiconductor optical pumps for fiber lasers or solid state lasers is that the lasing wavelength is now fixed by the crystal and not by the diode. Efficiency is also sacrificed, and the structure becomes large and cumbersome.
It is possible to attempt to increase the single spatial mode output power of semiconductor lasers, for example by phase-locking a number of individual emitters together (phased-arrays), or by using a semiconductor amplifier structure (MOPA—master oscillator and power amplifier). However, the arrays tend to lock out of phase, causing a dead spot in the center of the beam. In MOPA structures, the high optical density tends to distort the phase of the light and the spatial coherence. Furthermore, high power MOPA structures require flared amplifier sections that lead to a mode with very high astigmatism and asymmetry, further reducing their utility.
One can obtain higher output powers by combining the optical output of different lasers together, when each laser is designed to emit at different wavelength. In this way multiple laser outputs are combined with a wavelength selective element such as a diffraction grating or an arrayed waveguide grating (AWG). Light of different polarizations can also be combined in a loss-less fashion with a polarizing beam splitter, which increases the number of possible inputs by a factor of 2. For example for Raman pumping optical amplifiers, multiple 1450-1500 nm wavelengths are frequently combined into a single output. The problem with this approach is that each laser must be designed to a precise wavelength, and carefully coupled to multiplexer. The coupling loss between laser to fiber, and fiber to multiplexer and the loss of the multiplexer itself all reduce the total output power. With multiple optical couplings and precise wavelength lasers, the overall system becomes complex and expensive.
An alternate way of making a multi-wavelength system is to use multiple gain chips with a single wavelength selective element. The feedback to each gain chip occurs at a different wavelength and the outputs are automatically combined into one. There have been demonstrations of this approach using integrated InP technology. The problem is that the optical loss of the InP multiplexer is high. In such an external cavity configuration, the impact of this loss on laser power is higher than a loss that comes outside of the cavity. Furthermore, an integrated chip is large and has poor yield.